Fasting, abstinence from food or drink or both for health, ritualistic, religious, or ethical purposes. The abstention may be complete or partial, lengthy, of short duration, or intermittent. Fasting has been promoted and practiced from antiquity worldwide by physicians, by the founders and followers of many religions, by culturally designated individuals (e.g., hunters or candidates for initiation rites), and by individuals or groups as an expression of protest against what they believe are violations of social, ethical, or political principles.

Medical fasting

Fasting has been used therapeutically since at least the 5th century bce, when Greek physician Hippocrates recommended abstinence from food or drink for patients who exhibited certain symptoms of illness. Some physicians recognized a fasting instinct, whereby patients in certain disease states naturally experience a loss of appetite. Some physicians believed that administering food during such states was unnecessary and possibly even detrimental, since fasting was thought to be an important natural part of the recovery process.

An understanding of the physiological effects of fasting began to evolve in the latter part of the 19th century, when some of the first organized studies of fasting were carried out in animals and humans. In the 20th century, as more became known about nutrition and the nutritional requirements of the human body, methods of fasting became increasingly sophisticated, and a wide array of approaches emerged. For example, fasting was used as a treatment and as a form of disease prevention, and it was carried out in various settings (e.g., in a hospital or clinic or at home). Some fasting methods, particularly those applied in the treatment of chronic diseases, lasted more than a month, allowed for the consumption of only water or calorie-free tea, and included exercise and enemas. Other methods, generally referred to as modified fasting, allowed for the intake of 200 to 500 kilocalories per day (daily calorie needs of adults range from about 1,600 to 3,000 kilocalories, depending on sex, age, and activity level) and sometimes included psychological or spiritual therapy; depending on the particular method used, calories usually were in the form of bread, vegetable broth, fruit juice, honey, or milk. Modified fasting was distinguished from a very low-calorie diet, which allowed up to 800 kilocalories per day and typically was aimed at inducing substantial weight loss. Intermittent fasting involved cyclic periods of calorie restriction, such as a 24-hour period of fasting followed by a 24-hour period of regular calorie consumption.

By the 21st century, although fasting clearly was applicable in some instances of disease, such as in certain acute diseases (particularly when accompanied by a loss of appetite), whether fasting in other instances was beneficial to human health remained unclear. For example, whereas research in humans had suggested that intermittent fasting carried out over 15 days improved insulin-mediated glucose uptake into tissues, studies in rodents had indicated that such fasting, carried out over the long-term, promoted glucose intolerance and the release of damaging oxidants from tissues.

Fasting and religion

In the religions of ancient peoples and civilizations, fasting was a practice to prepare persons, especially priests and priestesses, to approach the deities. In the Hellenistic mystery religions (e.g., the healing cult of the god Asclepius), the gods were thought to reveal their divine teachings in dreams and visions only after a fast that required the total dedication of the devotees. Among the pre-Columbian peoples of Peru, fasting often was one of the requirements for penance after an individual had confessed sins before a priest. In many cultures the practice was considered a means to assuage an angered deity or to aid in resurrecting a deity who was believed to have died (e.g., a god of vegetation).

In the religions of some tribes of Native Americans, fasting was practiced before and during a vision quest. Among the Evenk of Siberia, shamans (religious personages thought to have the power to heal and to communicate psychically) often received their initial visions not with a quest but rather after an unexplained illness. After the initial vision, however, they fasted and trained themselves to see further visions and to control spirits. Historically, priestly societies among the Pueblo Indians of the American Southwest fasted during retreats before major ceremonies connected with seasonal changes.

Fasting for special purposes or before or during special sacred times remains a characteristic of major religions of the world. In Jainism, for example, fasting according to certain prescribed rules and practicing certain types of meditation leads to trances that enable individuals to dissociate themselves from the world and reach a transcendent state. Some Buddhist monks of the Theravada school fast as part of their meditation practices. In India, Hindu sadhus (holy men) are admired for their frequent personal fasts for various reasons.

Among the Western religions, only Zoroastrianism prohibits fasting, because of its belief that such a form of asceticism will not aid in strengthening the faithful in their struggle against evil. The other Western religions—Judaism, Christianity, and Islam—emphasize fasting during certain periods. Judaism, which developed many dietary laws and customs, observes several annual fast days, primarily on days of penitence (such as Yom Kippur, the Day of Atonement) or mourning. Christianity, especially Roman Catholicism and Eastern Orthodoxy, has observed a 40-day fast period during Lent, a spring period of penitence before Easter, and during Advent, a penitential period before Christmas. Among Roman Catholics the observance has been modified since the Second Vatican Council (1962–65) to allow greater individual choice, with mandatory fasting only on Ash Wednesday and Good Friday during Lent. Protestant churches generally leave the decision to fast to individual church members. The month of Ramadan in Islam is a period of penitence and total fasting from dawn to dusk.

Fasting as a form of protest

In addition to its role in religion, fasting may be used to express social and political views, particularly as a gesture of protest or solidarity. The classic example of this approach was set by Mahatma Gandhi, who in the early 20th century conducted a fast in prison to atone for the violent excesses of those of his followers who did not practice his teaching of satyagraha (nonviolence) against British rule in India. Gandhi later often fasted in pursuit of similar objectives, including the removal of disabilities imposed by the government on the untouchables. Fasting has frequently been practiced to protest against war and what are considered social evils and injustices, as in the fasts of the American black comedian Dick Gregory from the 1960s in protest against the violation of civil rights of American Indians and against U.S. military activity in Southeast Asia. In 1981, 10 Irish nationalists died in a Belfast prison during a hunger strike conducted to urge recognition of themselves and their associates as political prisoners.

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A deliberate self-denial of food and drink, usually for religious or ethical reasons, is called fasting. The word is probably derived from a Teutonic or Germanic term meaning "to be strict" or "to observe." The forgoing of food and drink may be partial or total and may last for a short or long period of time. In addition to fasting, many religions also have dietary restrictions: certain foods are not to be eaten by believers of a certain faith. Judaism and Islam, for instance, have regulations against the eating of certain kinds of meat, and in Islam drinking alcoholic beverages is also forbidden.